THE CONCRETE BRIDGE MAGAZINE

SPRING 2017

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Contents of this Issue

Navigation

Page 34 of 55

CONCRETE BRIDGE TECHNOLOGY
High-Strength Steel in Bridge Applications
by Dr. Maher K. Tadros, e.construct USA.LLC
The most common type of reinforcing bars used for passive reinforcement of concrete in the United States is Grade 60 ksi steel conforming to ASTM A615/A615M, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement. Grade 270 ksi, seven-wire prestressing strand, conforming to ASTM A416/A416M, Standard Specification for Low-Relaxation, Seven-Wire Steel Strand for Prestressed Concrete, has been used for prestressing, both pretensioning and post-tensioning. The strand grade has not changed for more than 40 years.
In the meantime, both concrete strengths and reinforcing bar strengths have experienced significant increases in the past 20 years. Concrete strength has nearly doubled from about 5 ksi to about 10 ksi in common practice. Concrete with strengths even as high as 22 ksi, which is known as ultra-high-performance concrete (UHPC), has been used in several projects.
A steel grade with a yield strength of 100 ksi was recently introduced into the ASTM A615 standard. Both the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318-14) and Commentary (ACI 318R-14) and the American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications have recognized the higher strength reinforcing bars and have allowed use of 100 ksi steel in some applications and 75 to 80 ksi in others. Over the past 15 years, a new type of steel, which combines high strength with corrosion resistance and conforms to ASTM A1035/A1035M, Standard Specification for Deformed and Plain, Low- Carbon, Chromium, Steel Bars for Concrete Reinforcement, has been recognized in North American building codes. The introduction of this corrosion-resistant high-strength steel into the AASHTO LRFD specifications was based on recommendations from National Cooperative Highway Research Program (NCHRP) Project 12-77 by Shahrooz et al.1 Figure 1 shows the stress-strain relationship for the various steel types.
It seems logical to try to combine highstrength concrete with high-strength steel to optimize structural members and systems; however, limitations exist. This article focuses on introducing corrosionresistant high-strength steel conforming to ASTM A1035/A1035M. Examples of successful applications are also given.
What Is ASTM A1035 Steel?
The proprietary high-strength steel meeting ASTM A1035 specifications is manufactured in the United States and other countries under license from MMFX Technologies, Irvine, Calif. ASTM A1035 has three basic series: 2000, 4000, and 9000, representing a chromium content of 2, 4, and 9%, respectively. The highest chromium content is the 9000 series, also known as ASTM A1035-CS alloy. It is the most expensive and is used for the highest corrosion-resistance applications, for example bridge decks. The 4000 series is known as ASTM A1035-CM alloy. The 2000 series, also known as ASTM A1035-CL alloy, is the least expensive and can be used where low-corrosion resistance is acceptable, for example columns (generally where they are under axial loads and flexural strains remain small) and interiors of buildings.
Strength and Ductility
ASTM A1035 also specifies other chemical composition requirements, most notably carbon content. The 2000, 4000, and 9000 series have maximum carbon contents of 0.30%, 0.20% and 0.15%, respectively. The chemical composition allows all three series to have a yield strength of at least 100 ksi and a tensile strength of at least 150 ksi. There are actually six series of ASTM A1035 (cont. next page)